Alcohol abuse and dependence are widespread and it is estimated that 14 million American adults abused alcohol or were dependent on it in 1992 and that approximately 10% of Americans will be affected by alcohol dependence sometime during their lives. Alcohol dependence, characterized by the preoccupation with alcohol use, tolerance, and withdrawal, is a chronic disorder with genetic, psychosocial, and environmental factors influencing its development and manifestations. Studies have demonstrated the significance of opioids (i.e., beta-endorphin), dopamine (DA), serotonin (5-HT), γ-amino-butyric acid (GABA) and glutamate for the development and maintenance of alcohol dependence.
Various medications and behavioral therapy have been used to treat alcohol dependence. The neuronal targets of alcohol include many neurotransmitter systems and the molecules participating in or regulating the systems, including GABA, glutamate, DA, opioids, and serotonin (for a review see Johnson, 2004, Expert Opin. Pharmacother., 5:9:1943-1955).
Despite the number of studies performed in this area, few drugs for alcohol dependence are approved in the U.S. The approved drugs are disulfiram, naltrexone, Vivitrex®/Vivitrol® (a long-acting depot formulation of naltrexone), and acamprosate. Disulfiram is an irreversible inhibitor of aldehyde dehydrogenase leading to increased levels of acetaldehyde, a toxic intermediate in alcohol metabolism. Patients who take disulfiram and drink alcohol experience an increased dilation of arterial and capillary tone producing hypotension, nausea, vomiting, flushing, headache and possibly in some, worse symptoms. Therefore, the concept behind the use of disulfiram is that the alcohol-dependent individual associates drinking with unpleasant adverse events and, as a result, avoids further alcohol consumption. Nevertheless, recent research shows that disulfiram has limited utility because compliance is low unless it is administered by a partner or spouse.
Serotonin (5-HT) dysfunction probably contributes to the development of alcoholism. Serotonin's receptors contribute to alcohol use in animals, as alcohol increases basal levels of 5-HT affecting receptors. Of the seven distinct families of 5-HT receptors, three are known to contribute to alcohol dependence: 5-HT1A receptors might be associated with alcohol consumption and the development of tolerance; 5-HT2 receptors with reward; and 5-HT3 receptors with the development of reinforcement. Based on such evidence, several serotonergic drugs have been examined, but with inconsistent results. Presently only sertraline and ondansetron (a serotonin-3 (5-HT3) antagonist) appear to show any promise with certain subtypes of alcoholic patients and fluoxetine with depressed alcoholics (see Kenna, 2005, Drug Discovery Today: Therapeutic Strategies, 2:1:71-78 and Johnson, 2000, Alcohol. Clin. Exp. Res., 24:1597-1601).
The 5-HT3 receptor is involved in the expression of alcohol's rewarding effects. Behavioral pharmacological studies show that many of alcohol's rewarding effects are mediated by interactions between DA and 5-HT receptors in the midbrain and cortex. 5-HT receptors are densely distributed in the terminals of mesocorticolimbic DA containing neurons, where they regulate DA release in these brain regions. These DA pathways, particularly those in the NAc, are involved in mediating the rewarding effects of abused substances including alcohol. Demonstration that 5-HT3 receptor blockade reduces DA activity, and therefore the rewarding effects of abused drugs (including alcohol), comes from at least three different animal paradigms. 5-HT3 receptor antagonists: 1) attenuate hyperlocomotion in the rat induced by DA or ethanol injection into the nucleus accumbens; 2) inhibit DiMe-C7 (a neurokinin)-induced hyperlocomotion, which is also attenuated by the DA antagonist, fluphenazine; and 3) decrease alcohol consumption in several animal models and across different species.
Animal studies demonstrated that the 5-HT3 receptor facilitates some of the biochemical and behavioral effects of alcohol through midbrain DA release. 5-HT3 antagonists are consistently shown to suppress alcohol preference in animal studies, with recent evidence suggesting the 5-HT3A receptor subunit requisite for 5-HT3 antagonist-induced reductions in alcohol consumption.
Ondansetron, a 5-HT3 receptor antagonist, has functionally opposite effects to SSRIs and blocks serotonin agonism at the 5-HT3 receptor. According to studies, ondansetron can be effective for early-onset alcoholics (EOA) but not late-onset alcoholics (LOA), where age of onset of alcoholism (younger versus older than 25 years old) is the basis for subtyping alcoholics (Johnson, 2000, Alcohol. Clin. Exp. Res., 24:1597-1601). In a placebo-controlled trial, 271 participants were stratified into EOA and LOA subtypes by 1, 4, and 16 μg/kg twice-daily doses of ondansetron compared with placebo (Johnson, 2000, J. Am. Med. Assoc., 284:963-971). Patients with EOA who received ondansetron showed significant reductions in drinking (particularly those receiving 4 μg/kg twice daily) compared with LOA across all groups. In another study, it was shown that ondansetron treatment is more likely to be associated with improved drinking outcomes among EOA compared with LOA (Kranzler et al., (2003, Alcohol. Clin. Exp. Res., 27:1150-1155). Ondansetron continues to be examined for individuals with early-onset alcoholism.
The reasons for these differential effects are unknown; however, one hypothesis suggests that alcoholics with a biological predisposition have a dysregulation of serotonergic function primarily associated with serotonin transporter (SERT) function (Johnson, 2000, Alcohol. Clin. Exp. Res. 24:1597-1601). The polymorphic variation of the SERT (the 5′-HTTLPR) is hypothesized to be involved with the effectiveness of ondansetron and sertraline in EOA and LOA alcohol-dependent individuals, respectively. Given that epidemiologic studies demonstrate that alcohol dependence has an approximately 50-60% heritability, the prospect for positive outcomes to drug therapy at least partly dependent on genetic predisposition in some alcoholics is strong. Recent studies have, therefore, attempted to delineate the genetic components associated with alcohol dependence. These findings highlight the role that 5-HT plays in alcohol consumption, although drug trials using serotonergics have had difficulty delineating responders from non-responders.
Vulnerability to alcohol dependence is heritable, with a rate ranging from 0.52 to 0.64 (Kendler, 2001). Despite this high heritability rate, only one marker allele (alcohol-metabolizing aldehyde dehydrogenase genes) has been identified consistently to be associated with alcoholism (Kranzler et al, 2002). Of the various neurotransmitter systems through which alcohol mediates its effects, the serotonergic system has been shown to play a role in alcohol preference and consumption (Johnson, 2004). Synaptic serotonergic neurotransmission is terminated when serotonin (5-HT) is transported back into pre-synaptic neurons by 5-HT transporters (5-HTTs) (Talvenheimo and Rudnick, 1980). Therefore, a major part of the functional capacity of the serotonergic system is regulated by the 5-HTT. Heavy episodic drinking is associated with numerous psychiatric and general medical conditions causing a major public health burden (Cargiulo, 2007). Several studies have reported a dose-response relationship between the extent of heavy drinking and the risk of alcohol related morbidity and mortality among heavy drinkers (Makela and Mustonen, 2007; Gastfriend et al., 2007). Consequently, reduction of heavy drinking is used as an indicator of treatment response in clinical trials aimed at treating alcohol dependence.
There is a long felt need in the art for compositions and methods useful for diagnosing, treating, and monitoring alcohol disorders and susceptibility to alcohol disorders.